Ion mobility spectrometry is a versatile technique used to detect presence of molecular species in a gas sample. The technique has particular application in detection of explosives, drugs, and chemical agents in a sample, although it is not limited to these applications. Portable detectors are commonly used for security screening, and in the defence industry. However, conventional portable devices are still nonetheless relatively large.
Ion mobility spectrometry relies on the movement of different ion species through an electric field to a detector; by appropriate selection of the parameters of the electric field, ions having differing properties will reach the detector at differing times, if at all. Time of flight (TOF) ion mobility spectrometry measures the time taken by ions when subject to an electric field to travel along a drift tube to a detector. Ions of different characteristics will reach the detector at different times, and the composition of a sample can be analysed. This form of spectrometry relies on the length of the drift tube for its resolution; the longer the drift tube, the more powerful the detector. This restricts the possible miniaturisation of such spectrometers, since there is a limit to the lower size of the drift tube which may effectively be used. Further, given that relatively high electric field strengths are necessary, the restriction on drift tube length also results in the need to use relatively high voltages in the device, which may be potentially hazardous to the operator and further restricts the possibility of miniaturisation of the device.
A variation on TOF ion mobility spectrometry is described in U.S. Pat. No. 5,789,745, which makes use of a moving electrical potential to move ions against a constant drift gas flow towards a detector. A plurality of spaced electrodes are alternately pulsed to generate a moving potential well, which carries selected ions along with it. This device is unsuited to miniaturisation due to, among other reasons, the need for a pump to produce the drift gas flow.
Field asymmetric ion mobility spectrometry (FAIMS) is a derivative of time of flight ion mobility spectrometry (TOFIMS), which potentially offers a smaller form factor; however, existing designs use moving gas flows and high voltages, which are undesirable for microchip implementations. Scaling is further hindered by molecular diffusion, an effect that becomes significant in the micron regime. Background information relating to FAIMs can be found in L. A. Buryakov et al. Int. J. Mass. Spectrom. Ion Process. 128 (1993) 143; and E. V. Krylov et al. Intl. Mass. Spectrom. Ion Process. 225 (2003) 39-51; hereby incorporated by reference.
Conventional FAIMS operates by drawing air at atmospheric pressure into a reaction region where the constituents of the sample are ionized. Chemical agents in vapour-phase compounds form ion clusters. The mobility of the ion clusters is mainly a function of shape and weight. The ions are blown between two metal electrodes, one with a low-voltage DC bias and the other with a periodic high-voltage pulse waveform, to a detector plate where they collide and a current is registered. Ions are quickly driven toward one electrode during the pulse phase and slowly driven toward the opposite electrode between pulses. Some ions impact an electrode before reaching the detector plate; other ions with the appropriate differential mobility reach the end, making this device a sort of differential mobility ion filter. A plot of the current generated versus DC bias provides a characteristic differential ion mobility spectrum. The intensity of the peaks in the spectrum, which corresponds to the amount of charge, indicates the relative concentration of the agent.
While this arrangement offers the possibility for greater miniaturisation than conventional TOFIMS, the need to generate a constant gas flow requires the presence of a pump, which limits the lower size of such a device. Representative examples of such devices are described in U.S. Pat. Nos. 6,495,823 and 6,512,224.
We have developed a further modification of PAWLS, which is described in international patent publications WO 2006/046077 and WO 2006/013396. The content of these publications is incorporated herein by reference. As described in these publications, the technique does not require a drift gas flow for its operation. Instead, an electric field is used to cause ions to move toward the detector. An ion filter having a plurality of ion channels formed by an interdigitated array of electrodes is used; the electrodes are operated so as to produce both a filter and a drive electric field. The drive field serves to drive ions through the filter, in place of a drift gas flow, while the filter field acts to permit only selected ions through the filter. This arrangement allows for a solid state construction which does not require a gas pump or similar, so allowing for greater miniaturisation of the device than would otherwise be possible, as well as a more robust construction.
We have now devised a modification of these devices, in which a pulsed drift gas flow is used. The pulsed drift gas flow has benefits in terms of pre-concentrating ions to be detected, and may be used to improve sensitivity. Further, a fixed mass of analyte introduced into the spectrometer will not be diluted as with a continuous drift gas flow, so increasing sensitivity. Relatively small pumps may be used to generate a pulsed flow, so maintaining the advantages of miniaturisation offered by the ion filter arrangement described above. The pulsed flow arrangement may be used with alternative ion filter geometries, such as those used in conventional FAIMS.